If I understand you right you have a banner and want to know which color to use as background color?
You can use Paint.net (
http://www.getpaint.net/) or any similar paint program:
- open the image
- select the color picker-tool
- click on the desired color in your banner, so it gets selected in the color dialog
- in the color dialog click on more, there is a field "Hex", use that value as background color